This invention relates to a method of boiling liquefied gas in a heat exchanger (or the like) by heat exchange with another fluid, and to a heat exchanger for carrying out such boiling of liquefied gas. The invention also relates to a condenser-reboiler suitable for use in a rectification column in which a gas mixture, for example air, is separated.
In order to boil a liquefied gas it is necessary for the surface from which heat is transferred to the boiling liquid to be at a higher temperature than the liquid. It is known that the temperature differential "delta T" between the surface and the liquid required to effect boiling at a given rate is dependent on the nature of the surface: if the surface is smooth and planar the necessary temperature differential needs to be greater than if the surface is, for example, relatively rough. The reason for this phenomenon is apparently that, for example, a rough surface provides many more and better sites at which vapour bubbles can be nucleated than does a smooth, planar surface. Once a bubble is formed at the surface it grows to a radius sufficiently large for it to break away from the nucleating site, and either travels upwards along the surface or merges with another bubble at a nucleating site thereabout. As the bubbles travel upwards so they coalesce with other bubbles that lie in their path and the enlarged bubbles continue to move upwards.
It has therefore been proposed to form the heat transfer surfaces of heat exchangers for use in boiling liquefied gases with other than a smooth regular planar surface finish. For example, U.S. Pat. No. 3,301,314 discloses a heat transfer wall having formed therein a plurality of indentations of microscopic dimension whose depth is greater than their maximum width and which are partially filled with a deposit of a lower surface energy material, said material having a contact wetting angle with the liquid being boiled of at least 80.degree.. Typically, the material of low surface energy is polytetrafluoroethelyene. U.S. Pat. No. 3,384,154 discloses bonding layers of porous material to the heat exchange surfaces of a heat exchanger. The pores, which are the size of capillaries, act as nucleation sites. It has also been proposed to roughen the heat exchanger surfaces of a heat exchanger by scratching such surfaces.
We believe, however, that there is a limit to the reduction in "delta T" that can be achieved merely by providing nucleation sites for the formation of bubbles. This is because once a bubble has grown to a size sufficiently large for its buoyancy to cause it to leave or "break away" from a nucleation site, further energy will be required to nucleate another bubble at that site.
In UK patent specification No. 1 304 861 there is described a heat conductive base member for transferring heat from a heat source on one side thereof to a boiling fluid on the other side thereof: a plurality of spaced apart fins having substantially smooth and uninterrupted side surfaces extending from said other side of said base member, each of said fins having a base portion joined to said base member and a tip portion bent over toward the next adjacent one of said fins to form a continuous gap between said tip portion and said one fin, said gaps having a width from 0.001 to 0.005 inches, the gap between said tip portion and said next adjacent one of said fins being less than the space between the respective base portions of adjacent fins whereby a continuous re-entrant shaped cavity is formed between adjacent ones of said fins.
In use, superheated liquid is trapped between the fins, and forms bubbles. The bubbles grow in size until their buoyancy is sufficient to overcome the surface tension at the tips of the fins when the bubbles will break away. A substantial mass of vapour remains within the elongate cavities between the fins and liquid enters the cavity to take the place of the space occupied by the vapour that has "broken" away in the form of bubbles. The incoming liquid displaces the vapour along the groove such that new bubbles form at different sites.
In U.S. Pat. No. Re. 30,077 there is described a heat exchange wall having a boiling surface layer formed thereon with a plurality of cavities. The cavities are adapted to entrap vapour bubbles within the boiling surface layer to provide boiling nucleation sites.
Each cavity is open to the boiling surface layer through a restricted opening which has a cross-sectional area smaller than the largest cross-sectional area in the cavity interior. The opening provides egress for vapour from the interior of the cavity to the boiling surface layer during boiling.